REVY, ISM / SCIENCE PHOTO LIBRARY REVY, ISM / SCIENCE PHOTO LIBRARY
Blood group test showing reactions between blood types (columns) and antibody serums (rows). There are four human blood types (top to bottom): A, B, AB and O. Type A blood has type A antigens (surface proteins) on its blood cells and anti-B antibodies (proteins that bind to type B antigens) in its serum. Type B blood is the reverse. Mixing type A blood with anti-A serum (left column) causes an agglutination reaction, producing dense red dots. Type B blood undergoes the same reaction with anti-B serum (middle column) AB blood has no antibodies, but has both A and B antigens on its cells, so it agglutinates in the anti-A+B serum (right column). Type O blood has both antibodies but no antigens, so it does not react to the serums.
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